1. Field of the Invention
The invention relates to an exhaust gas control apparatus and a reductant dispensing method for an internal combustion engine.
2. Description of the Related Art
A known technology related to this invention is a urea SCR system in which a selective catalytic reduction type NOx catalyst (hereinafter, referred to as “SCR catalyst”) that removes NOx from exhaust gas by adsorbing NH3 (ammonia) is disposed in an exhaust passageway of an internal combustion engine, and in which a reductant dispensation portion supplies a reductant to the SCR catalyst. In a urea SCR system disclosed in Japanese Patent Application Publication No. 2003-293737 (JP-A-2003-293737), the amount of consumption of NH3 adsorbed to the SCR catalyst is found on the basis of the amount of NOx discharged from the internal combustion engine and a NOx removal rate of the SCR catalyst that is derived by a removal rate derivation portion. The adsorbed amount of NH3 adsorbed on the SCR catalyst is found according to the amount of consumption of NH3 found as described above and the dispensed amount of the reductant supplied to the SCR catalyst. Then, on the basis of the thus-found the adsorbed NH3 amount and a predetermined target adsorbed NH3 amount that is slightly lower than a saturated adsorbed NH3 amount on the SCR catalyst, the dispensation of the reductant is controlled.
However, the present inventors have discovered that the state of adsorption of NH3 on the SCR catalyst includes a weakly adsorbed state and a strongly adsorbed state. The weakly adsorbed state is a state in which the NH3 adsorbed on the SCR catalyst is useful for the reduction reaction of NOx. On the other hand, the strongly adsorbed state is a state in which the NH3 adsorbed on the SCR catalyst is not useful for the reduction reaction of NOx unless the state is changed into the weakly adsorbed state. Therefore, it has come to be understood that in the case where the dispensation of the reductant is controlled on the basis of the amount of NH3 adsorbed on the SCR catalyst as in the urea SCR system disclosed in Japanese Patent Application Publication No. 2003-293737 (JP-A-2003-293737), the NOx removal rate cannot be increased, if the state of adsorption of NH3 on the SCR catalyst is not taken into account but only the total adsorbed amount of NH3 on the SCR catalyst is taken into account. Thus, in order to increase the NOx removal rate, the reductant dispensation control needs to be performed in light of the adsorbed state of NH3 suitable for removal of NOx.